[YAML Parser] Add more csharp keywords and fix messages that has field with value to use static type instead of const type

[wijanarko-sukma]

• Add "namespace", "const" and "static" for csharp keywords
• Fix on message generation to use static type (it should have been enum) for any field with value

[andreaskoepf]

I integrated the proposed changes as part of https://github.com/Xamla/ROS.NET/commit/dacfe85a3a5e3f932bf1c127ec2211465f5ffc6d